Detector for frequency modulated signals



March 18, 1947. J. c; TELLIER 2,417,717

DETECTOR F0113 FREQUENCY MODULATED SIEGNALS Filed 0ct. 24, 1944 2 Sheets-Sheet l All INVENTOR. v 4

WW I By/ose ob C. 7l/ier ATTORNE Y5 18, 1947. J. c. TELLIER DETECTOR FOR FREQUENCY MODULATED SIGNALS Filed 001:. 24, 1944 n Fig.2

2 Sheets-Sheet 2 mulmmmuuuumwm INVENTOR. Joseph C- 715/6/" A TTORNE Y5 Patented Mar. 18, 1947 DETECTOR FOR FREQUENCY MODULATED SIGNALS Joseph C. Tellier, Penn Wynne, Pa., assignor, by mesne assignments, to Philco Corporation, Philadelphia, Pa., a corporation of Pennsyl- Vania Application October 24, 1944, Serial No. 560,129

9 Claims. 1

The present invention relates to radio apparatus and particularly to a detector of frequency modulated signals.

More particularly still the invention provides a detector of frequency modulated signals which is insensitive to amplitude modulated signals and does not require balancing, limiting or the like.

The present circuit operates to count the number of cycles of intermediate frequency, frequency modulated, signals occurring in a fixed time interval and derives from this counted numher a signal which is similar to the modulating signal at the transmitter.

It is an object of the present invention to provide a detector of frequency modulated signals which shall be insensitive to amplitude modulated signals and which requires no balancing or the like.

It is a further object of the invention to provide such a detector which operates by counting the number of cycles of I. F. frequency modulated signals occurring during a fixed time interval.

It is a further object of the invention to provide such an FM detector which is simple and inexpensive to construct and is stable in operation.

Other objects and features of the invention will be apparent when the following description is considered in connection with the annexed drawings in which:

Figure 1 is a schematic diagram of the circuit of the FM detector of my invention;

Figure 2 is a curve showing the voltage applied to the detector plotted as a function of time; and

Figure 3 is a curve showing the output of the detector as well as the derived audio signal both plotted as a function of time.

Referring now to the drawings, the intermediate frequency signal from the last intermediate amplifier is applied to the control grid of the pentode through the coupling coil I and associated circuit elements. The pentode II has a sharp cut-01f and the time constant of the condenser resistance combination |2-|3 is such that the average value of the bias applied to the pentode is equal to the peak value of the intermediate frequency. Consequently, if the peak value of the intermediate frequency signal is greater than the cut-ofi bias of the tube the pentode conducts during a portion only of the positive half of the intermediate frequency cycle. As a consequence, the output from the pentode is represented by Figure 2 and is a wave which is square topped and the amplitude of which is constant.

As shown in Figure 2 the square topped wave is composed of pulses of unequal duration and of uneven spacing, since, with the system described above both the duration of the pulses and the spacing between would vary. It should, however, be understood that a wave having impulses of equal duration can readily be produced by known methods and that therefore the curve of Figure 2 represents but one wave form of output from the pentode which is useful in the detector of my invention.

The detector of my invention comprises a multivibrator generally designated at M together with the diode rectifiers I5 and I6 and the various associated circuit arrangements about to be described. The multivibrator comprises two triodes i7 and iii together with proper interconnections which are well known in the art. The multivibrator has a fixed frequency, for example 25 kc., and is preferably adjusted so that triode I! is in the conducting condition for longer periods than in the non-conducting condition, e. g. for or more, of the operating cycle.

The details of the circuit can best be described by considering the operation thereof. Let it first be assumed that the tube II, that is the right hand triode of the multivibrator as seen in Figure 1, is conducting, then the plate of tube H and consequently the lower plate of condenser 23 will be substantially at ground potential. With this condition in mind it will be seen that while the plate voltage of the pentode II is 250 volts, that is while the pentode I I is driven below plate current cut-off value, the condenser 2| charges to 250volts through the diode It. When the plate voltage of pentode I drops to approximately zero, the point designated 23 tends to go to a minus 250 volts. Under these circumstances, tube l6 ceases to conduct but tube I5 becomes conductive and consequently, condenser 2| discharges into condenser 26 through the tube [5.

If the condenser 20 has a capacity many times (for example 2500 times) that of condenser 2| then a small portion, in this case approximately of the total voltage or 1 1; of a volt will appear across condenser 26?, the upper plate being negative with respect to the lowerplate.

When the plate of pentode again has 250 volts thereon condenser 2| is recharged. However, condenser 2t cannot discharge because tube It is non-conductive during this interval. Then for each positive cycle of intermediate frequency the negative voltage across condenser 20 is increased by a definite amount which in the example heretofore given isfi; of a volt.

Thus the voltage occurring between the point designated 26 and ground will be as shown in Figure 3, and the higher the intermediate frequency the greater the number of pulses which will be counted during the given time interval, '6, and consequently, the greater the amplitude of the voltage at point 2d. For example, if the intermediate frequency varies from4'.225 me. to-

4375 me. the number of pulses counted during each 25 kc. interval, or each 1/1 0 of a second, will vary from 170 to 175.0. Clearly then the voltage appearing at the point 24 is an audio voltage proportional to the frequency of the signal, with intermediate frequency and 25 kc. components. However, the use of the filter comprising the coil 25 and condenser 26 serves to remove all but the audio components and therefore an audio wave, as indicated by the dash line of Figure 3, results.

It will, of course, be understood that both the intermediate frequency and the multivibrator frequency may be varied between wide limits but must always be in substantial excess of the upper limit of audibility.

While I have described a preferred embodiment of my invention it will be clear that the principle thereof may be utilized in other. circuit arrangements and the values of the circuit elements may be varied without departing from my invention and therefore I wish the scope of my invention to be determined not by the foregoing description but solely by the appended claims.

What is claimed is:

1. In a detector of frequency modulated signals, in combination, means to convert said signals into square-topped signals of constant amplitude and of spacing inversely proportional to the frequency of the frequency modulated signals, a source of timing signals, and means for applying said square topped signals and said timing signals to an output circuit for roducing therein a signal having an audio frequency component of magnitude proportional to the number of square topped impulses produced during each interval determined by said timing circuit.

2. In a detector of frequency modulated signals, in combination, means to convert said signals into square topped signals of constant amplitude and of spacing inversely proportional to the frequency of the frequency modulated signals, a multivibrator, the two sections of which are alternately rendered conducting, said alternation occurring at a predetermined rate, and means for producing a signal having an audio frequency componet of a magnitude proportional to the number of said impulses produced during the interval of conduction of one of said multivibrator sections.

3. In a detector of frequency modulated signals, in combination, means to convert said signals to square topped impulses of constant amplitude and of spacing inversely proportional to the frequency of said frequency modulated signals, a capacity, means to charge said capacity upon each increase in voltage of said square topped impulses, a multivibrator comprising two triodes alternately rendered conductive at a predetermined rate, a capacity connected to the plate of one of said triodes, means to discharge said 4 first mentioned capacity into said second mentioned capacity upon each decrease in voltage of said square topped wave and means to discharge said second mentioned capacity when the second triode becomes conductive.

4. In a detector of frequency modulated signals, in combination, a pentode tube having a sharp cutoff characteristic, means to apply the frequency modulated signals to the control grid of said pentode, a capacity connected to the plate circuit of said pentode, said capacity being charged to the plate voltage of said pentode when said pentode is driven below cutoff, a multivibrator comprising two electronic tubes alternately rendered conducting, a second capacity connected to the plate of one of said multivibrator tubes and means for discharging said first mentioned ca- 1 pacity into said second mentioned capacity when said pentode conducts whereby the charge of said second capacity is increased by equal increments upon the reception of each cycle of frequency modulated signal and means to discharge said second mentioned capacity upon the other of said multivibrator tubes becoming conducting whereby signals are produced having an audio component proportional to the number of cycles of frequency modulated signal received during a predetermined time interval.

5. In a detector of frequency modulated signals, in combination, a pentode tube having a sharp cutoff characteristic, means to apply the frequency modulated signals to the control grid of said pentode, and a capacity connected to the plate circuit of said pentode, said capacity being charged to the plate voltage of said pentode when said pentode is driven below cutoff, a multivibrator comprising two electronic tubes alternately rendered conducting, a second capacity connected to the plate of one of said multivibrator tubes and means comprising a pair of oppositely poled rectifiers, one of which forms a portion of the charging circuit for said first mentioned capacity and the other of which forms a portion of a circuit for discharging said first mentioned capacity into said second mentioned capacity when said pentode conducts whereby the charge or" said second capacity is increased by an equal increment upon the reception of each cycle of frequency modulated signal, and means to discharge said second mentioned capacity upon the other of said multivibrator tubes becoming conducting whereby signals are produced having an audio component proportional to the number of cycles of frequency modulated signal received during a predetermined time interval.

6. In a detector of frequency modulated signals, in combination, a pentode tube having a sharp cutoff characteristic, means to apply the frequency modulated signals to the control grid of said pentode, and a capacity connected to the plate circuit of said pentode, said capacity being charged to the plate voltage of sa1d entode when said pentode is driven below cutoil, a multivibrator comprising two electronic tubes alternately rendered conducting, a second capacity connected to the plate of one of said multivibrator tubes and means comprising a pair of oppositely poled rectifiers, one of which forms a portion of the charging circuit for said first mentioned capacity and the other of which forms a portion of a circuit for discharging said first mentioned capacity into said second mentioned capacity when said pentode conducts whereby the charge of said second capacity is increased by an equal increment upon the reception of each cycle of frequency modulated signal, means to disharge said second mentioned capacity upon the other of said multivibrator tubes becoming conducting, and means to supply the audio frequency components of the signals formed by said charging and discharging of said second mentioned capacity to an output circuit.

'7. In a detector of frequency modulated signals, in combination, a pentode tube having a sharp cutoff characteristic, means to apply the frequency modulated signals to the control grid of said pentode, and a capacity connected to the plate circuit of said pentode, said capacity being charged to the plate voltage of said pentode when said pentode is driven below cutoff, a multivibrator comprising two electronic tubes alternately rendered conducting, a second capacity connected to the plate of one of said multivibrator tubes and means comprising a pair of oppositely poled rectifiers, one of which forms a portion of the charging circuit for said first mentioned capacity and the other of which forms a portion of a circuit for discharging said first mentioned capacity into said second mentioned capacity when said pentode conducts whereby the charge of said second capacity is increased by an equal increment upon the reception of each cycle of frequency modulated signal, means to discharge said second mentioned capacity upon the other of said multivibrator tubes becoming conducting, and means comprising an inductance and capacity to supply the audio frequency components of the signals formed by said charging and discharging of said second mentioned capacity to an output circuit.

8. In a detector for frequency modulated signals, in combination, means to convert said signals into square topped signals of constant amplitude and of spacin inversely proportional to the frequency of the frequency modulated signals, electronic switching means including means to render a circuit alternately conducting and non-conducting, and means for producing a signal having an audio frequency component of magnitude proportional to the number of square topped impulses produced during each conductive interval determined by said electronic switching means.

9. In a detector for frequency modulated signals, in combination, means to convert said signals into square topped signals of constant amplitude and of spacing inversely proportional to the frequency of the frequency modulated signals, a capacity, means to charge said capacity upon each increase in voltage of said square topped impulses, a source of time interval signals, a capacity connected to said time interval signal source, means t discharge said first mentioned capacity into said second mentioned capacity upon each decrease in voltage of said square topped wave, and means to discharge said second mentioned capacity upon the completion of each tim interval.

JOSEPH C. TELLIER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,286,377 Roberts June 16, 1942 2,307,316 Wolff Jan. 5, 1943 

